1. Field of the Invention
The present invention relates to a novel semiconductor laser device. It provides a semiconductor laser device in which longitudinal and transverse modes are stabilized.
2. Description of the Prior Art
Semiconductor laser devices have many advantageous properties such as small size, operation at a high efficiency, and the capability of direct modulation by a drive current. They are, therefore, prospective light sources for optical communications, optical information processing systems, etc.
Among various semiconductor lasers, the distributed-feedback semiconductor laser has a corrugated surface therein. It achieves mode stabilization by exploiting a sharp oscillation mode selectivity which is produced by the diffraction effect of light due to the corrugated surface.
Distributed-feedback semiconductor laser devices have been reported in detail in "Appl. Phys. Lett.,", Vol. 27, pp. 403-405, October 1975, by M. Nakamura et al, and in "IEEE J. Quantum Electron" Vol. QE-12, pp. 597-603, October 1976, by K. Aiki et al. A typical example of a distributed-feedback semiconductor laser is one disclosed in U.S. patent application Ser. No. 512,969, filed Oct. 7, 1974, now U.S. Pat. No. 4,178,604, issued Dec. 11, 1979.
FIGS. 1 and 2 of the accompanying drawings show an example of a distributed-feedback semiconductor laser device. FIG. 1 is a sectional view taken orthogonally to the traveling direction of light, while FIG. 2 is a sectional view taken along the traveling direction of light. Here, numeral 1 designates an n-GaAs substrate, numeral 2 an n-Ga.sub.1-x Al.sub.x As layer (x.apprxeq.0.3), numeral 3 a GaAs active layer, numeral 4 a p-Ga.sub.1-x Al.sub.x As layer, and numeral 5 periodic corrugations which are provided at the boundary between the semiconductor layer 3 and the semiconductor layer 4. Numerals 6 and 7 indicate ohmic electrodes. The layers 2, 3, and 4 constitute an optical waveguide. Light generated in the active layer 3 is centered around the layer 3, and is subjected to a Bragg reflection of 180.degree. by the periodic corrugations 5 formed at the boundary between the layers 3 and 4.
A distributed-feedback semiconductor laser device oscillates at a single longitudinal mode and exhibits a spectral width of 0.5 A or less, so that it has excellent monochromaticity. In addition, the temperature-dependency of the oscillation wavelength is low.
Even in a distributed-feedback semiconductor laser device controlled to have a single longitudinal mode, however, the generation of excess optical noise, especially for a modulated signal, due to transverse mode instability cannot be prevented.
On the other hand, a buried heterostructure semiconductor laser device has been proposed as a typical semiconductor laser device. This structure is provided with an optical waveguide which produces the effect of confining lateral carriers and light in the direction of the thickness of an active layer. An example of a buried heterostructure semiconductor laser device of this sort is described in pending U.S. patent application Ser. No. 786,758, filed Apr. 2, 1974, assigned to Hitachi, Ltd.
Even in buried heterostructure semiconductor laser devices oscillating at a single transverse mode, however, the generation of the excess optical noise due to mode interaction is unavoidable.